1. Field of the Invewntion
This invention relates to infrared and visible radiation detectors in general and more particularly to a filter for use with infrared and visible sensors or detectors, specifically a filter which transmits short wavelength electromagnetic radiation and rejects, through reflection and absorption, long wavelength radiation.
2. Description of the Prior Art
The infrared and visible portions of the electromagnetic spectrum have become increasingly important for the collection and analysis of information for many applications, for example the remote sensing of weather information and tactical data acquisition. Infrared and visible detectors and sensors are frequently utilized in applications and environments in which they may be subject to unwanted, interfering or even damaging electromagnetic radiation, particularly from sources which emit electromagnetic radiation at microwave frequencies. It is known to place filter elements, i.e. windows, in front of infrared and visible detectors or sensors to shield them from damage by rejecting electromagnetic radiation falling outside of the frequency range of interest. Ideally, such a filter should have its pass and rejection characteristics matched to the spectral response of the infrared device that it is to be used with. That is, it should transmit infrared radiation within the frequency range of interest with minimum absorption or reflection loss and reject, through reflection and absorption, electromagnetic radiation outside of the band of interest, including signals at microwave frequencies. In the case of an ideal filter, there should be a sharp transition i.e., rolloff, between the wavelength of the signals which the filter passes and the wavelength which it rejects. In addition, the filter should have high transmission in the pass band and high rejection in the rejection band.
Filter elements which have heretofore been used with infrared sensors and detectors include windows manufactured from semiconducting materials, for example silicon or germanium. While filters of the foregoing type provide some rejection of microwave signals, they are not ideal because the attenuation that windows manufactured from semiconducting materials provide at microwave frequencies is either a constant or decreases with increasing wavelength. As a result, the windows must either be thicker or have higher conductivites than desired in order to meet microwave attenuation requirements. Still another form of prior known filter consists of an insulating window made from zinc sulfide or zinc selenide or glass on which a conducting wire mesh is deposited. While windows of the foregoing configuration are useful for passing infrared and visible radiation and for rejecting, by reflection or absorpotion, radiation outside of the infrared or visible regions of the electromagnetic spectrum, for example signals at microwave frequencies, they do not exhibit the characteristics of an ideal filter, that is, they do not exhibit a steep rolloff between signals within the frequency band that they pass and those signals falling outside the band of interest. In the case of a window having a mesh deposited on it, the microwave attenuation of the window exhibits a gradual increase with increasing wavelength, i.e. outside of the infrared or visible portions of the electromagnetic spectrum. Also, the attenuation provided at microwave frequencies by meshes deposited on insulating substrates is reduced at certain frequencies because of interference between the front and back surfaces of the substrate. Thus, the attenuation provided by the filter is dependent on the thickness of the substrate and the frequency of the miccowave signal striking the window. The pass/rejection characteristics of windows manufactured from semiconducting material are also inferior to those of an ideal filter. While a window manufactured from a semiconductor material will exhibit a relatively high attenuation of signals at wavelengths longer than infrared, it exhibits a decrease in attenuation of such signals as their wavelength increases (i.e. for microwave signals of lower frequency). Consequently, neither a mesh filter nor a semiconductor substrate along provide the desired response characteristics for an idealized short wavelength pass filter. The invention disclosed herein seeks to overcome the aforementioned deficiencies by providing a window filter, particularly useful for infrared and visible sensors and detectors, which provides broadband rejection of signals at microwave frequencies while minimizing the loss of signals at infrared and visible frequencies.